The present invention relates to a mass spectrometer including an ion trap, and a mass spectrometric analysis method performed in the mass spectrometer.
A mass spectrometer is an analytical instrument which charges sampled molecules of a measured object, and measures the mass number to electrical charge ratio of the generated ions, and an ion current, as mass spectral data. Since a substance possesses a specific molecular mass related to the kinds and number of atoms composing the substance, by obtaining the mass spectral data of the substance, it is possible to obtain important information to specify the substance. Further, since a mass spectrometer can be directly connected to a chromatography apparatus such as a gas or liquid chromatograph measurement device, a capillary electrophoresis chromatograph measurement device, and so forth, it can be used as a detector of high sensitivity, which is capable of obtaining various kinds of qualitative information, for a chromatography apparatus.
Recently, concern for environmental problems, health of a human body, etc. has grown, and a chromatograph direct-coupled type mass spectrometer has been used in various fields such as in checking for harmful organic compounds included in clean water, waste water, foods, etc., assessing the safety of a developed new drug, and so forth. Requirements for such an analytical instrument are as follows.
(1) It has a high sensitivity, and can perform a quantitative analysis with the analytical instrument.
(2) It is possible to obtain qualitative information enough to specify a substance certainly.
(3) It can remarkably reduce the work for a sample preparation, which is necessary in the measurement of a sample, to remove impurities which are not to be analyzed.
(4) Since a pesticide to be analyzed includes ten to sixty components, the analytical instrument is required simultaneously to analyze as many substances as possible by a single sample injection.
Specifically, in order to achieve the requirement (2), mass spectral data of standard substances are measured and stored in advance. On the other hand, a mass spectrum of the substance corresponding to each peak which has appeared in a chromatograph of a sample, is obtained, and a standard substance which has a mass spectrum coinciding with the measured mass spectrum corresponding to each in the chromatograph of the sample, is searched. Further, it is determined that if there are substances which have the mass spectra highly coinciding with the measured spectra of the peak substances in the chromatograph of the sample, respectively, the sample includes these substances.
In comparison to examine the coincidence between mass spectra of standard substances and those of respective peak substances in a sample, it frequently occurs that mass spectra of peak impurity substances included in the sample are measured, the impurity substances being not includes in the standard substances, and this causes deterioration of the measurement accuracy in the comparison results. If an atmospheric-pressure ionization method is used for ionizing a sample to be analyzed by a mass spectrometer, the obtained mass spectrum is generally a simple spectrum in which the peak corresponding to the molecular number of a sample has the highest intensity. Although the feature of the atmospheric-pressure ionization method is effective to confirm the molecular number of a sample, since there is generally a smaller number of peaks due to bond-cleavage ions in a mass spectrum obtained by using this ionization method in comparison with that obtained by using an electron impact ionization method, the mass spectrum obtained by using the atmospheric-pressure ionization method gives poor information to identify a sample, in the case where the identification of the sample is performed by comparing measured mass spectra of the sample with those of standard substances. Generally, the atmospheric-pressure ionization method is used for an ionization chamber connected to a liquid chromatograph, that is, an LC (Liquid Chromatograph)/MC (mass spectrograph) system, and the electron impact method is used for an ionization chamber connected to a gas chromatograph, that is, GC (Gas Chromatograph)/MC (mass spectrograph) system. Here, in the LC/MC system, after obtaining a mass spectrum for a definite range of mass only, it is very difficult to specify components of a sample.
To solve the above problem, what has been performed is, components of a sample are identified by specifying a representative mass number of each component of the sample and performing MSn analysis with regard to ions of the mass number, in order to increase spectral information on the sample.
In Japanese Patent Application Laid-Open Hei. 10-142196, it is disclosed that MS1 analysis and MSn analysis are carried out in succession.
In the MSn analysis, ions of a specified mass number in ions, which have been introduced into a mass spectrograph, are selected. Then, energy is given to these selected ions by making them collide with neutral particles, and they are cleaved. Further, respective groups of the cleaved ions are sent to a detector in order of the mass number of the groups, and mass spectral data of each group of ions is obtained. Mass spectral data obtained by sending ions, which have been introduced into the mass spectrograph, to the detector without causing any reaction in these ions, is called simply MS spectral data, or MS1 spectral data. On the other hand, mass spectral data of the above cleaved ions which have received the cleavage reaction of one stage, is called MSn spectral data. Moreover, by sending the selected ions to the detector after causing cleavage reactions in the selected ions, which have been introduced into the mass spectrograph, in multiple stages, data of MS3, MS4, or MS5, can be obtained. Since there is a portion in a molecule in which a cleavage easily occurs due to the structure of the molecule, even if the molecular number of a sample is equal to that of another sample, it is possible to distinguish samples of the same molecular number by comparing the respective spectral data of ions of the samples of different structures, which have received the cleavage reactions of multiple stages. Therefore, even if spectral information enough to specify a sample cannot be obtained by using usual MS data, the sample can be identified by using MSn data. Further, since the specific kind of ions are selected and cleaved, and MS2 spectrum of the cleaved ions is obtained, the peaks existing in the spectral data, which are caused by impurities, can be excluded.
Although the above MSn analysis is excellent with regard to exclusion of influences due to impurities, and improvement of accuracy in substance-specification, it is necessary to specify the mass number of target ions to be cleaved, in advance, in order to perform the MSn analysis. If the analysis concerning a known substance is performed, it is easy to specify the mass number of target ions to be cleaved. However, if a sample whose components are unknown is analyzed, since the mass number of ions to be cleaved cannot be specified, it is necessary to perform the MSn analysis by determining the mass number of the ions to be analyzed, after obtaining total spectral information on the sample, by measuring MS1 spectral data for a predetermined range of mass number once only.
In such an analysis, the task of the user is heavy, and the analysis takes much time. Particularly, in the LC/MC system using the atmospheric ionization method, since the MSn analysis is almost indispensable for the substance-specification, it is desirable that the MSn analysis be smoothly or efficiently performed.
Moreover, the kinds of ionized components of a sample, which have been introduced into a mass spectrograph from a chromatograph, change moment by moment. Therefore, the kind of ionized component which is analyzed by the MSn analysis has already changed from that of the ionized component which should have been analyzed by the MSn analysis, in measurement processing in which the MSn analysis is performed after MSn spectral data for a predetermined range of mass number once only, which in turn causes erroneous results of the analysis. That is, it occurs that the analysis performed by the mass spectrograph cannot follow changes in the kinds of ionized components in the sample.
The present invention has been achieved in consideration of the above-described problems, and is aimed at providing a mass spectrometric analysis method, and an apparatus implementing the method, which is capable of accurately specifying substances included in a sample by using mass spectral data obtained by MS2 analysis.
To achieve the above objective, the present invention provides a spectrometric analysis method and an apparatus implementing the method, in that a data base storing spectral data sets of respective standard substances, which are obtained by analyzing these standard substances in advance, each spectral data set including a name of a substance, its MS1 spectral data and MS2 spectral data, is constructed; and MS2 spectral data sets obtained by MS2 analysis of a sample are compared with MS2 analysis spectral data sets stored in the data base.